1. Field of the Invention
The present invention relates to an optical system for taking image, and more particularly to a miniaturized optical system for taking image used in a PC camera.
2. Description of the Prior Art
In recent years, with the popularity of electronic image taking equipment, such as the PC camera, and the camera mobile phone, the optical system for taking image has been becoming thinner and thinner, and the photosensitive assembly of an ordinary digital camera is nothing more than CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor). Due to the advancement of the semiconductor technique, the pixel size of photosensitive assembly has been being reduced continuously, and the development of the miniaturized optical system for taking image is toward the high resolution field. Therefore, the demand for the image quality becomes increasingly urgent.
A common optical system for a PC camera is usually arranged in such a manner that the front lens group is negative and the rear lens group is positive, namely, a so-called inverse telephoto structure, so as to obtain a wide field of view. And to reduce the incident angle of the light with respect to the photosensitive assembly, the refractive power of the rear lens group should be improved. Usually, an ordinary rear lens group consists of two plano-convex lens elements with convex surfaces facing each other, this is a so-called RAMSDAN structure. The front lens group has negative refractive power and the rear lens group has positive refractive power, and this is an inverse telephoto. The conventional optical system here particularly refers to RAMSDAN structure of the inverse telephoto, and this structure has the following disadvantages:
The incident angle of the light with respect to the front surface of the second lens of the rear lens group is too large, and as a result, the sensitivity will be increased comparatively, and the light at this position is likely to be divergent. Therefore, the system will not be stable.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.